1. Field of the Invention
The present invention relates to a friction clutch, in particular for a motor vehicle, and relates more particularly to a clutch which is equipped with a device for compensating for the wear which is due, in particular, to wear in at least one friction liner, the said device, hereinafter referred to as a wear take-up device, operating in accordance with the wear that takes place in the liner or liners.
2. Description of Related Art
A conventional friction clutch generally includes a reaction plate, which may be divided into two parts so as to constitute a damping flywheel or a flexible flywheel, which is mounted on, and rotatable with, a first shaft which is conventionally a driving shaft such as the crankshaft of the internal combustion engine, and supporting through its outer periphery a cover plate to which at least one pressure plate is attached.
The pressure plate is fixed to the cover plate and reaction plate for rotation with them, while being displaceable axially under the biasing action of axially acting clutch engaging means controlled by declutching means; the clutch engaging means may consist of helical springs, or of one or two Belleville rings mounted in series or in parallel and subjected to the action of declutching levers which constitute the declutching means; generally, the clutch engaging means and the declutching means are both part of the same component, for example a metallic diaphragm which bears on the cover plate; the diaphragm may be mounted in series or in parallel with a Belleville ring for providing assistance to the declutching force.
A friction disc, carrying at least one friction liner at its outer periphery, and being fixed to and rotatable with a shaft which is conventionally a driven shaft such as the input shaft of the gearbox, is interposed between the pressure plate and the reaction plate so as to be gripped between them when the clutch is in its engaged condition. The clutch engaging means control the axial displacement of the pressure plate when they are actuated by a clutch release bearing through the declutching means. Most commonly, the friction disc comprises an axially resilient support disc, on either side of which there are fixed two friction liners. The friction disc is thereby gripped progressively. In another version the support disc is rigid.
In the course of the working life of such a clutch, the friction liner or liners and the components with which they cooperate, namely the pressure plate and reaction plate, become worn, and this causes variation in the position of the pressure plate and the positions of the axially acting clutch engaging means and the clutch release bearing, from which it follows that there is a variation in the gripping force between the friction disc on the one hand and the pressure and reaction plates on the other, because the working conditions of the declutching means are modified, and the force necessary to disengage the clutch is thereby affected. The equipping of such a clutch with a wear take-up device avoids these disadvantages, and the clutch engaging means, as well as the clutch release bearing, which are conventionally in constant engagement on the declutching means, occupy the same position when the clutch is in its engaged condition.
It is known to provide a friction clutch, in particular for a motor vehicle, of the kind comprising a reaction plate adapted to be mounted on a driving shaft for rotation with the latter, a friction disc, carrying at its outer periphery at least one friction liner and arranged to be mounted on a driven shaft for rotation with the latter, a pressure plate, a cover plate fixed on the reaction plate, axially acting clutch engaging means which are controlled by declutching means and which act between, firstly, the cover plate and, secondly, the pressure plate through secondary abutment means and primary abutment means, the pressure plate being fixed with respect to the cover plate for rotation with the latter while being displaceable axially with respect to it and being subjected to the action of resilient return means biasing the pressure plate axially towards the cover plate, the said clutch further including a wear take-up device including ramp means, the ramps of which are disposed circumferentially, the ramp means being located axially between the abutment means and the pressure plate and adapted to cooperate with counter ramp means, a set of teeth with which a tangentially disposed worm cooperates, the ramp means being fixed to the said set of teeth for rotation therewith, means being provided for driving the worm in rotation and being rendered operational by wear in the friction liner or liners when the clutch is engaged, the said worm driving means comprising a ratchet wheel fixed in rotation to the worm and controlled through a control tongue, the course of travel of which, with respect to the ratchet wheel during the declutching operation, is limited by a so-called control abutment, the worm and the means for driving the worm in rotation being carried by a support member which is fixed to the cover plate.
A clutch of this kind is described for example in the document FR-A-2 753 503.
In that document, the clutch engaging means consist of a diaphragm which is in cooperation with a primary abutment carried by the base portion of the cover plate, and which has at its outer periphery a radial finger through which it is caused to cooperate with the control tongue; the control abutment which limits the course of travel of the control tongue is carried by the support member fixed to the cover plate.
As has already been mentioned above, the wear take-up device ensures that the clutch engaging means, in this case the diaphragm, will be in the same position of engagement regardless of the state of wear, namely the position which it occupied in the new state when it was fitted on the vehicle; it is well understood to be important that this position, which is called the nominal position, should correspond to the exertion of a desired force, referred to as the nominal force, by the diaphragm on the pressure plate in the engaged condition; the position of the diaphragm, which in practice means its inclination, is clearly fixed by those of the primary and secondary abutments on which its outer portion, in the form of a Belleville ring, is in engagement; the primary abutment is carried by the cover plate and the position of the secondary abutment is a function of the positioning of the ramp means with respect to the counter-ramp means. The positioning is obtained by the regulation function, which is itself provided by the assembly consisting of the ratchet wheel, the worm and the control tongue which is adapted for cooperation with the control abutment.
This assembly is part of a cassette which is provided with a specific support member for mounting the above mentioned assembly.
For putting the diaphragm in its nominal position, the mechanism is associated with a virtual friction disc by simulating wear of the liners, and a certain number of clutch engagement and disengagement operations are carried out until the control tongue, engaging on the control abutment, no longer jumps over a tooth of the ratchet wheel, the secondary abutment of the diaphragm being then correctly positioned; thus, the nominal position of the diaphragm depends on both the primary abutment and the control abutment.
As has been seen, the mechanism of the clutch is constructed by assembling various components together; these components are manufactured, and their assembly is carried out, with certain tolerances; this affects the precision of the relative positioning of the primary abutment and control abutment when these latter are formed on two different components as is the case in the document cited above, in which the primary abutment is carried by the cover plate and the control abutment by the support member assembled to the cover plate.
An object of the present invention is to overcome this drawback.
Thus, according to the invention, a clutch of the type described above is characterised by the fact that the control abutment, like the primary abutment means, is arranged on the cover plate.
This makes it easy to obtain precise positioning of the control abutment with respect to the primary abutment means, especially where the primary abutment means are made in the form of circumferential elements press-formed in the base portion of the cover plate, the primary abutment means and the control abutment being formed in the same operation of fabricating the cover plate.
Preferably, the control abutment consists of at least one lug formed by stamping in the base portion of the cover plate. Ventilation of the clutch is improved by the hole formed in pressing out the lug.
In another version, the control abutment consists of at least one boss press-formed in the base portion of the cover plate. This operation is very easy to perform. In all cases, it is of advantage to provide, for a given cover plate and different applications, a means for enabling the height (i.e. the size) of the control abutment to be adjusted so as to obtain different adjustments of the position of the diaphragm.
Such a means may be formed directly in the press by partial modification of the press and calibrating tools.
In another version this is achieved by machining the height of the control abutment.
Preferably, the axially acting clutch engaging means consist of a diaphragm.
In a preferred embodiment, the primary abutment means consist of a circumferential press-formed element, of continuous or divided form, in the base portion of the cover plate; in this case, the common fabrication operation in which the cover plate is formed is a press-forming operation where the control abutment is a press-formed element.
Preferably, the ratchet wheel is controlled by the diaphragm through the control tongue; the ratchet wheel is controlled by an actuator element consisting of a finger formed on the diaphragm at its outer periphery; two lugs or bosses are provided, extending radially and spaced apart circumferentially by a passage for the actuator element.
In another embodiment, the control abutment consists of a lug formed by stamping out in the base portion of the cover plate, which lug includes an end portion which extends axially and passes through a hole in the diaphragm.
Preferably, the ramp means consist of an annular ring which includes, axially on one side, the ramps, which are inclined and spaced apart circumferentially, and, on the other side, the secondary abutment means.
Preferably, the counter-ramp means consist of pads adapted to cooperate with the ramps of the ramp means.
Advantageously, the worm is mounted for displacement along its axis, being subjected to the action of a resilient so-called take-up means.
In order to enable the subject matter of the invention to be understood, embodiments which are shown in the attached drawings will now be described, by way of purely illustrative and non-limiting example.